This invention relates to an electrical resistance device and, more particularly, to a resistance device having the specific property of exhibiting a sharp increase in its electrical resistance as the temperature increases in a narrow temperature range (PTC, i.e, positive temperature coefficient, characteristics).
Materials having PTC characteristics can be utilized in a control device by which heat generation is ceased when a heater reaches a high temperature in a PTC thermisor, in a heat-sensitive sensor; and in a protection device. In these devices, when an excessive current flows through a circuit due to a short or the like, the current increases and therefore self-heating is developed by Joule heat. The PTC characteristics of the materials cause the resistance to increase, restricting the current to a predetermined value or less, so that when the short is released the circuit is restored. A variety of materials has been developed having PTC characteristics. For example, materials having PTC characteristics are ceramic-type materials comprising BaTiO.sub.3 having a monovalent or trivalent metal oxide incorporated therein; and a polymer-type material comprising a polymer such as polyethylene having an electrically conductive material such as carbon black dispersed therein.
As shown in FIG. 3, a PTC device generally comprises a material having PTC characteristics 2 consisting of a polymer having an electrically conductive material dispersed therein (a PTC composition), metallic electrode plates 3a and 3b having the PTC composition sandwiched or interposed therebetween, and lead plates 4a and 4b connected to the electrode plates 3a and 3b, wherein each electrode plate is connected to a separate device, apparatus, power source or the like via each lead plate.
The PTC device is obtained by first preparing a PTC composition, forming this PTC composition into a film, hot pressing metallic foil electrodes to upper and lower surfaces of the film to form a laminate, cutting this laminate to a predetermined size, and providing a lead plate on the surface of each of the electrodes by soldering, welding or the like. The joining between the PTC composition and the electrode plates is carried out by hot pressing the PTC composition to the electrode plates at a temperature close to the melting point of the PTC composition.
It is desirable that the PTC device exhibit as low a resistance value as possible at room temperature (room temperature resistance) and as high a resistance value as possible at a high temperature (a peak resistance). The room temperature resistance is primarily dependent on the type of the PTC composition and the adhesion between the PTC composition and the surface of each of the electrodes. In order to reduce the room temperature resistance, the amount of the electrically conductive particles packed in the PTC composition can be increased. However, in this case, the peak resistance is decreased and therefore it is impossible to obtain a high ratio of peak resistance to room temperature resistance. In order to improve adhesion between the PTC composition and the surface of each of the electrodes, a process for decreasing the contact resistance between the PTC composition and each of electrodes has been proposed (U.S. Pat. Nos. 4,238,812 and 4,426,339).
In electrically connecting the lead plates to the electrodes of the PTC device by soldering, welding or the like, the PTC composition which is in contact with the electrode plates is heated, and a portion of the PTC composition is pyrolyzed by this heat to evolve decomposed gases. Further, a portion of the PTC composition is evaporated to evolve vapors. Thus, the adhesion between the PTC composition and the electrodes is impaired, increasing the contact resistance therebetween.